Remove v1 -> v2 migrations

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

This removes a ton of code that was effectively dead. The migration now provides some help for users trying to go directly from <1.8.0 to 3.0.0.

Closes #306

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

There's reasonable risk here. My hope is that almost everyone using the tool has upgraded to 1.8.0 or later already, in which case this will have no impact for them.

QA

How did you test this?

I started writing some unit tests, but I couldn't figure out a way to mock the dialog interaction. Since this is one-time code that will never need to be changed again, there isn't a huge amount of value in unit testing anyway.

Instead, I tested the 4 possible scenarios manually:

• Upgrading from 2.0 to 3.0. Everything is smooth, request history is still there
• Upgrade from 1.7 to 3.0 and declining the upgrade
  • It quits with an error message
• Upgrade from 1.7 to 3.0 and accept
  • It wipes out the old and starts up fine
  • Confirmed I'm able to make new requests too
• Running on 3.0 with no upgrade

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked